ALBERT

Description: We have measured the carbon isotopic composition of dissolved inorganic carbon in bottom waters of the Ontong Java Plateau (western equatorial Pacific) and on the northern Emperor Seamounts (northwest Pacific). Each of these locations is several hundred miles from the nearest Geochemical Ocean Sections Study (GEOSECS) stations, and the observed delta13C values at each site differ substantially from regionally averaged GEOSECS delta13C profiles. We discuss the possible causes of these differences, including horizontal variability, near-bottom effects, and problems with the Pacific GEOSECS delta13C data. We also measured the isotopic composition (C and O) of core top C. wuellerstorfi from a depth transect of cores at each location. The delta18O data are used to verify that our samples are Holocene. Comparison of foraminiferal and bottom water delta13C values shows that this species faithfully records bottom water delta13C at both sites and demonstrates that there is no depth-related artifact in the dissolved inorganic carbon-C. wuellerstorfi delta13C relationship at these sites.

Description: Summary: The stratigraphy of the Shackleton Range established by Stephenson (1966) and Clarkson (1972) was revised by results of the German Expedition GEISHA 1987/88. The "Turnpike Bluff Group" does not form a stratigraphic unit. The stratigraphic correlation of its formations is still a matter of discussion. The following four formations are presumed to belong to different units: The Stephenson Bastion Formation and Wyeth Heights Formation are probably of Late Precambrian age. The Late Precambrian Watts Needle Formation, which lies unconformably on the Read Group, is an independant unit which has to be separated from the "Turnpike Bluff Group". The Mount Wegener Formation has been thrusted over the Watts Needle Formation. Early Cambrian fossils (Oldhamia sp., Epiphyton sp., Botomaella (?) sp. and echinoderms) were found in the Mt. Wegener Formation in the Read Mountains. The Middle Cambrian trilobite shales on Mount Provender, which form the Haskard Highlands Formation, are possibly in faulted contact with the basement complex (Pioneers and Stratton Groups). They are overlain by the Blaiklock Glacier Group, for which an Ordovician age is indicated by trilobite tracks and trails, low inclination of the paleomagnetic field and the similarity to the basal units of the Table Mountain Quartzite in South Africa. The Watts Needle Formation represents epicontinental shelf sediments, the Mount Wegener Formation was deposited in a (continental) back-arc environment, and the Blaiklock Glacier Group is a typical molasse sediment of the Ross Orogen.

Description: The Recent distribution of living and dead benthic foraminifera of the Arctic Ocean proper has been examined in surface sediments that were sampled during the International Arctic Ocean Expedition 1991 (Arctic 91). The samples represent the Amundsen and Nansen Basins, the Morris Jesup Rise, and the Yermak Plateau from 90°N to 79°42.4'N, 05°15.6'E. Due to the technical difficulties of deep-sea drilling in the Arctic Ocean these areas have, until now, been investigated only in very low density sampling. The Arctic 91 sites of this study cover a water depth range between 552 and 4375 m and represent three sites which are seasonally ice-free, although not yearly, while the other sites are characterized by permanent sea-ice. There is a Recent production of benthic foraminifera in the whole investigation area and all surface samples contain both benthic and planktonic foraminifera. Abyssal assemblages are recorded in the Amundsen and Nansen Basins where Stetsonia arctica dominates with high abundances. It is, however, also possible to distinguish these two basins by the use of diagnostic species. At intermediate water depths (500 to 2000-2500 m) the faunas show higher diversities and higher abundances of Atlantic species than the deep-sea sites. Mixing of North Atlantic water down to approximately 2500 m, is suggested to explain the influx of Atlantic species on the Yermak Plateau and the Morris Jesup Rise. The foraminiferal tests are well preserved within the investigation area and dissolution does not seem to be very obvious in the deeper areas. There is no evidence from the Recent foraminiferal faunas that the bottom waters of the eastern, central Arctic Ocean are undersaturated with respect to calcium carbonate and the deep-sea areas appear, therefore, to lie above the present CCD.

Description: The neodymium isotopic composition of the silicate fraction of Holocene pelagic sediments from the North Pacific define two provinces: a central North Pacific province characterized by unradiogenic and remarkably homogeneous end (-10.2 +/- 0.5) and a narrow circum-Pacific marginal province characterized by more radiogenic and variable end (-4.2 +/- 3.8). The silicate fraction in the central North Pacific is exclusively eolian; based on prevailing wind patterns, meteorological data, and neodymium isotopic data, the only significant sediment source is Chinese loess. Leaching experiments on Chinese loess confirm that leachable Nd is isotopically indistinguishable from bulk and residual silicate Nd. Silicates in the circum-North Pacific marginal province comprise eolian loess, volcanic ash, and hemipelagic sediments derived from volcanic arcs. A compilation of Pacific seawater and Mn nodule epsilon-Nd data shows no clear spatial variation except for a general decrease from surface to deep waters from -3 to -4 and slightly lower epsilon-Nd in bottom waters along the western North Pacific due to the incursion of Antarctic Bottom Water. The relative homogeneity of bottom water epsilon-Nd, which contrasts sharply with the distinctive variation in sediment epsilon-Nd, plus the large difference between the average end of bottom waters and the central North Pacific eolian silicates (-4 vs. -10), suggests that any contribution of REE to seawater from eolian materials is insignificant. Furthermore, leaching of REE from eolian particles as they sink though the water column must be insignificant because Nd in shallow waters is more radiogenic than Nd in deeper waters. That there is no contrast in the Nd isotopic composition of bottom waters that overlie the central and marginal sediment provinces suggests that the ash and hemipelagic sediments derived from Pacific rim volcanic arcs also contribute minimal REE to seawater. The elimination of eolian, ash, and hemipelagic sediments leaves only near-shore riverine particulates as a possibly significant particulate source of REE to seawater.

Description: Near Weissbach - Diessbach southeast of Lofer (Salzburg, Austria), at the western rim of the Steinernes Meer (Northern Calcareous Alps), the sedimentary sequence of a local Jurassic strike slip basin (pull apart basin) is exposed. This basin formed during the Hettangian (Lias) along E-W directed sinistral strike slip faults showing the structure of a halfgraben with a complex basin fill and asymmetric facies distribution. Along the scarp faults, some of which are still well exposed, different types of mass flows were deposited. In the more distal part red bedded liassic limestones with numerous turbidite and debris flow intercalations (Weissbach Formation) are developed. These slope sediments are overlain by large, up to several hundred meter thick synsedimentary Dachsteinkalk-blocks which slid into the basin and mark a phase of extreme high subsidence. The blocks are overlain by thin, grey and red liassic limestones with a hardground and manganese horizon on top. At places hardground and manganese horizon directly rest on top of the blocks and debris flows. The limestones of the slope facies contain a rich and interesting holothurian fauna, which differs significantly from that of the German Lias. Among the numerous holothurian sclerites there are some stratigraphically important and new species like Theelia liassica MOSTLER, Theelia multiundulata MOSTLER (Upper Hettangian-Lower Pliensbachian), Theelia loferensis MOSTLER (Pliensbachian) und Neomicroanlyx ingridae MOSTLER (Lower Toarcian). Laterally the slope facies interfingers with and is overlain by sediments of the Allgäu Formation, which also contains a manganese mineralization. Apart from the scarp faults the Liassic sequence is developed in a different facies: Dachsteinkalk is overlain by a thin sequence of red and grey, nodular cherty limestones, which upward grade into well bedded cherty limestones and into sediments of the Allgäu Formation. Within the sedimentary sequence of the basin manganese mineralisations occur in two different stratigraphic horizons: A manganese mineralization together with the hardground on top of the Dachsteinkalk-blocks and debris flows and overlying condensed limestones (Lower Toarcian), and a stratiform manganese mineralization within the Allgäu Formation (Aalenian-Bajocian). This stratiform mineralization should not be mixed up with manganese nodule mineralizations which are very common in the deep sea environment. Three different orefacies types have been recognized. Cu-, Ba- and SiO2 -content of the manganese ores and the occurring manganese ore sludges indicate a local source from hydrothermal solutions which ascended along a deep fault system.

Description: Over the past decade an increasing body of evidence has accumulated indicating that much, perhaps most, of the deep sea floor is an environment of substantial temporal variability (Smith and Baldwin, 1984 doi:10.1038/307624a0; Smith, 1987; Deuser and Ross, 1980 doi:10.1038/283364a0; Thiel et al., 1988). This variability is driven largely by seasonal changes of processes occurring in the surface waters (Smith, 1987; Deuser and Ross, 1980; Billett et al., 1983 doi:10.1038/302520a0). The coupling of the deep sea floor environment to the surface waters is the result of rapid vertical transport of particulate matter through the water column (Honjo, 1982 doi:10.1126/science.218.4575.883; Deuser et al., 1986 doi:10.1016/0198-0149(86)90120-2; Lampitt, 1985 doi:10.1016/0198-0149(85)90034-2), affording only limited time for degradation before arrival at the sea floor. Studies in the Pacific Ocean have indicated that temporal variations in particulate organic carbon fluxes to the sea floor are accompanied by temporal variability in sediment oxygen demand by as much as a factor of four (Smith and Baldwin, 1984; Smith, 1987). We report here time-series studies of oxygen fluxes into the sediments of the oligotrophic Atlantic near Bermuda which contrast sharply with these previous reports. At the Bermuda site, despite large seasonal variations in particulate organic carbon fluxes, in situ measured sediment oxygen consumption does not vary significantly. These results imply that large areas of the sea floor may be characterized by seasonally invariant sediment oxygen demand.

Description: Results of studies in two biogeochemically active zones of the Atlantic Ocean (the Benguela upwelling waters and the region influenced by the Congo River run-off) are reported in the book. A multidisciplinary approach included studies of the major elements of the ocean ecosystem: sea water, plankton, suspended matter, bottom sediments, interstitial waters, aerosols, as well as a wide complex of oceanographic studies carried out under a common program. Such an approach, as well as a use of new methodical solutions led to obtaining principally new information on different aspects of oceanology.

Description: The formation of marine opal-CT nodules or layers as early diagenetic deposits has been documented only in Antarctic deep-sea sediments. In contrast, porcellanites and cherts in land sections and Deep Sea Drilling Project and Ocean Drilling Program drill sites are usually found in sediment sections of Miocene age and older. During R.V. Polarstem cruises ANT-IX/3 and 4, young porcellanites were recovered for the first time in contact with their host sediment in two cores from the Atlantic sector of the southern ocean. Chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation. In both cores the porcellanites are embedded in sediments rich in opal-A with extremely low levels of detrital minerals, an environment that seems conducive to a rapid transformation of biogenic silica into porcellanites.

Description: Because of a close relationship between detrital flux variations and magnetic susceptibility (MS) flux (MS cm**3 of bulk sediment multiplied by the linear sedimentation rate) variations in the southeast Indian basin of the southern ocean, MS flux profiles have been used to examine the spatial and temporal detrital flux changes in this basin during the last climatic cycle. Results indicate a general increase in detrital material input during the coldest periods, suggesting a widespread phenomenon, at least on the basin scale. Mineralogical data, geochemical data, and 87Sr/86Sr isotopic ratios have been used to determine the origin and transport mechanisms responsible for increased detrital flux during glacial periods. Mineralogical and geochemical data show that these glacial 'highs' are due to increases in both Kerguelen-Crozet volcanic and Antarctic detrital inputs. The 87Sr/86Sr isotopic composition of the 〉45-µm fraction indicates that the Kerguelen-Crozet province contributes to at least 50% of the coarse particule input to the west. This contribution decreases eastward to reach less than 10%. These tracers clearly indicate that the Crozet-Kerguelen province was a major source region of detrital in the western part of the basin during glacial times. In contrast, material of Antarctic origin is well represented in the whole basin (fine and coarse fractions). Because of the minor amount of coarse particles in the sediments, volcanic particles from Kerguelen and crustal particles from Antarctica have most probably been transported by the Antarctic bottom water current and/or the Circumpolar deepwater current during glacial periods as is the case today. Nevertheless, the presence of coarse particles even in low amount suggests also a transport by ice rafting (sea-ice and icebergs), originated from both Kerguelen and Antarctic sources. However, the relative importance of both hydrographic and ice-rafting modes of transport cannot be identified accurately with our data. During low sea level stands (glacial maximum periods), increasing instability and erosion of the continental platform and shallow plateaus could have resulted in a more efficient transfer of crustal and volcano-detrital material to the Southeast Indian basin. At the same time, extension of the grounded ice shelves over the continental margins and increase in the erosion rate of the Antarctic ice sheet could have induced a greater input of ice rafted detritus (IRD) to southern ocean basins. Enhancement of the circumpolar deepwater current strength might have also carried a more important flux of detrital material from Kerguelen. However, an increase in the bottom water flow is not necessarily required.

Description: Carbon isotopically based estimates of CO2 levels have been generated from a record of the photosynthetic fractionation of 13C (epsilon p) in a central equatorial Pacific sediment core that spans the last ~255 ka. Contents of 13C in phytoplanktonic biomass were determined by analysis of C37 alkadienones. These compounds are exclusive products of Prymnesiophyte algae which at present grow most abundantly at depths of 70-90 m in the central equatorial Pacific. A record of the isotopic compostion of dissolved CO2 was constructed from isotopic analyses of the planktonic foraminifera Neogloboquadrina dutertrei, which calcifies at 70-90 m in the same region. Values of epsilon p, derived by comparison of the organic and inorganic delta values, were transformed to yield concentrations of dissolved CO2 (c e) based on a new, site-specific calibration of the relationship between epsilon p and c e. The calibration was based on reassessment of existing epsilon p versus c e data, which support a physiologically based model in which epsilon p is inversely related to c e. Values of PCO2, the partial pressure of CO2 that would be in equilibrium with the estimated concentrations of dissolved CO2, were calculated using Henry's law and the temperature determined from the alkenone-unsaturation index UK 37. Uncertainties in these values arise mainly from uncertainties about the appropriateness (particularly over time) of the site-specific relationship between epsilon p and 1/c e. These are discussed in detail and it is concluded that the observed record of epsilon p most probably reflects significant variations in Delta pCO2, the ocean-atmosphere disequilibrium, which appears to have ranged from ~110 µatm during glacial intervals (ocean 〉 atmosphere) to ~60 µatm during interglacials. Fluxes of CO2 to the atmosphere would thus have been significantly larger during glacial intervals. If this were characteristic of large areas of the equatorial Pacific, then greater glacial sinks for the equatorially evaded CO2 must have existed elsewhere. Statistical analysis of air-sea pCO2 differences and other parameters revealed significant (p 〈 0.01) inverse correlations of Delta pCO2 with sea surface temperature and with the mass accumulation rate of opal. The former suggests response to the strength of upwelling, the latter may indicate either drawdown of CO2 by siliceous phytoplankton or variation of [CO2]/[Si(OH)4] ratios in upwelling waters.

Description: Early Cretaceous dinoflagellate cysts were reinvestigated from nine deep-sea sites of the North and Central Atlantic. In general the zonation scheme developed for the western Central Atlantic (Habib, 1977; Habib and Drugg, 1983 ) can also be applied to the eastern Central Atlantic. Comparison with the probabilistic zonation of Gradstein et al. (1992) show, however, that the first occurrences of the important marker species Druggidium apicopaucicure, Druggidium deflandrei, Druggidium rhabdoreticulatum and Odontochitina operculata appear to occur slightly later in the eastern Central Atlantic in respect to nannofossils and benthic foraminifers. Muderongia neocomica has a shorter stratigraphic range in the eastern Central Atlantic than in the western Central Atlantic.

Description: In recent years, temporal fluctuations in the abundance of C. d. davisiana have been used frequently as a highresolution stratigraphic and paleoenvironmental tool. The modern ecology and morphologic variation (temporal and geographic) of this radiolarian species is evaluated to ascertain its potential stratigraphic and paleoenvironmental significance. Statistics were obtained on the width and height of all C. d. davisiana segments from Pleistocene populations of differing ages from the Northern Hemisphere (Labrador Sea and Iceland-Faeroe Ridge) and Southern Hemisphere (Namibian shelf and Meteor Rise). Results reveal that segment height variations between and within populations are more conservative than segment width. The mean sizes of the thorax and first abdominal segment have distinguishable differences between C. d. davisiana found in the North and South Atlantic. All populations have no significant difference in first abdominal segment width, however, mean heights of this segment differ greatly between populations of the North and South Atlantic. Second abdominal segment sizes show no clear population grouping. Size differences in post-cephalic segment size of these populations would appear to be related to some isolation of gene pools and possibly unknown paleoenvironmental factors. Temporal changes in the postcephalic size of C. d. davisiana may be used to: (1) identify temporally equivalent peaks in abundance of the species in a given region, (2) possibly evaluate the degree of mixing of water'masses between regions, and (3) trace the initial spread of the species from its area of origin. Cleve's 1887 plankton samples, between Greenland and Spitzsbergen, were studied and used in conjunction with other data to make the following conclusions on the modern ecology of C. d. davisiana in the Arctic and Greenland-Norwegian Seas. (1) It is presently absent in surface water plankton samples, (2) it currently lives at depths below 500 m, where it is rare, (3) it does not live in the upper 200 m under Arctic ice but is rare at greater depths, (4) it is absent in the upper 200 m near permanent Greenland Sea ice where normal oceanic salinity prevails, and (5) it is most common in deep marginal fjord environments which may serve as a refuge for the species during interglacial periods. In the Atlantic Ocean, the abundance of C. d. davisiana does not exceed 1% of the assemblage between the Subtropical Convergence of each hemisphere. In the Norwegian and Labrador Seas the species may occasionally be in the range of 1-5% of the modern radiolarian assemblage and never more than 5% in the southern high latitudes. Apparently only in the modern Sea of Okhotsk, does the species presently occur in high abundance. We concur with Morley and Hays (1983) that increased abundances are likely caused by the development of a strong low-salinity surface layer associated with seasonal sea ice melting and a strong temperature minimum above warmer and higher salinity intermediate waters. Similar conditions were frequent during the Pleistocene in the high latitudes and its modern scarcity outside the Sea of Okhotsk must be related to the absence of the presently unique conditions in the latter region.

Description: High-resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the delta18O record indicates that the 41-kyr period of Earth's orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earth's climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y. The tuned ages for the magnetic boundaries Gauss/Gilbert and Top Cochiti are about 6-8% older than the ages of the conventional timescale. A major phase of Pliocene northern hemisphere ice growth occurred between 3.15 Ma and 2.5 Ma. This was marked by a gradual increase in glacial Atlantic delta18O values of 1per mil and an increase in amplitude variations by up to 1.5 per mil, much larger than in the Pacific deepwater record (site 846). The first maxima occured in cold stages G6-96 between 2.7 Ma and 2.45 Ma. Prior to 3 Ma, the isotope record is characterized by predominantly low amplitude fluctuations (〈 0.7 per mil). When obliquity forcing was at its minimum between 4.15 and 3.6 Ma and during the Kaena interval, delta18O amplitude fluctuations were minimal. From 4.9 to 4.3 Ma, the delta18O values decreased by about 0.5 per mil, reaching a long-term minimum at 4.15 Ma, suggesting higher deepwater temperatures or a deglaciation. Deepwater cooling and/or an increase in ice volume is indicated by a series of short-term delta18O fluctuations between 3.8 and 3.6 Ma.

Description: Statistical analysis of X-ray fluorescence data acquired during Leg 135 indicates that this instrument produces data of comparable precision to good land-based laboratories. We also examined contamination of certain elements caused by crushing during the use of the tungsten carbide apparatus. Although the concentrations of most elements are not altered during crushing, the powders prepared on the ship should not be used in subsequent studies where key elements of the investigation include W, Co, Ta, Pb, and low levels of Nb.

Description: New Pb, Sr, and Nd isotope data are presented for 64 samples from the six backarc sites drilled during Leg 135. Systematic changes in Pb and Sr compositions illustrate significant isotopic variations between and within sites as well as provide two key pieces of information. First, a recent influx of asthenosphere with Indian Ocean mantle affinities has occurred and has successfully displaced older "Pacific" asthenosphere from the mantle underlying the backarc region. Second, clear evidence exists for mixing between these two asthenospheric end-members and at least one "arc-like" component. The latter was not the same as most material currently erupting in the Tofua Arc, but it must have had a more radiogenic Pb-isotope signature, perhaps similar to rocks analyzed from the islands of Tafahi, and Niuatoputapu. A comparison between the isotopic variations and the tectonic setting of the drill sites reveals consistent and important information regarding the mantle dynamics beneath the evolving backarc basin. We propose a model in which the source of upwelling magmas changes from Pacific to Indian Ocean asthenosphere with the propagation of seafloor spreading, a model with important implications for the rate of mantle influx into this region. Although the chemistries of backarc magmas have been profoundly influenced by this process, an additional consequence is the advection of Indian Ocean asthenosphere into the sub-arc mantle source. The isotopic compositions of arc rocks from the vicinity have been reevaluated on the basis of the proposed mantle advection model. We suggest that the slab-derived flux of trace elements into the arc wedge has remained relatively uniform with time (i.e., ~40 Ma), so that the change in arc chemistry results from mantle source substitution, rather than from differences in the composition of the downgoing plate.

Description: Four petrographic lava types occur, ranging from aphyric to moderately phyric clinopyroxene-olivine tholeiitic basalts (Unit 1); olivine-clinopyroxene picritic basalts, sparsely to strongly olivine-phyric (Unit 3-type); olivine-clinopyroxene basalts (clinopyroxene dominant) (Unit 4); and moderately to strongly phyric two-pyroxene-plagioclase basaltic andesites (Unit 9-type). The olivine phyric lavas contain forsteritic olivines (extending to Fo92), and very magnesian Cr-rich spinels similar to those occurring in boninitic lavas. The basaltic andesites are mineralogically and petrographically indistinguishable from the modern Tofua Arc basaltic andesites, one notable feature being the highly calcic cores in plagioclase phenocrysts (up to An95). The forsteritic olivines, the Cr-spinels, and the calcic plagioclases are unlikely to have been precipitated in the lava compositions in which they occur, and are thought to have been incorporated from highly primitive melts by way of mixing processes (as advocated by Allan, this volume). Notwithstanding the evidence for mixing, the major element chemistries of the Unit 1- and Unit 9-type lavas are shown to be consistent with the derivation of the Unit 9-type basaltic andesites by means of fractional crystallization, through magmas of similar chemistry to Unit 1. Some trace element discrepancies in the modeling, and the relative volcanic stratigraphy of Site 839, however, preclude a direct liquid line of descent between the actual recovered units. Trace element data as well as TiO2 and Na2O data clearly illustrate the arc-like affinities of the magmas, with strong highfield-strength element depletion and large-ion-lithophile element enrichment. The abundance patterns are very close to those of the Tofua and Kermadec arc magmas, and also Valu Fa. Pb-, Sr-, and Nd-isotopic compositions indicate closest affinities with a "Pacific" MORB source, apparently characteristic of the western, older part of the Lau Basin. A subduction-related isotopic contribution is, however, inferred. The sources of the Site 839 magmas are thus inferred to be similar to, but less depleted geochemically, than those of the modern Tofua Arc magmas. The Site 839 sequence is interpreted as an older remnant of a volcanic construct of the "proto-Tofua arc", originally developed adjacent to the Tonga Ridge. Opening of the eastern Lau Basin, because of southward migrating propagators, has split and isolated the sequence, leaving it stranded within the modern Lau Basin.

Description: New major, trace element, and isotope data (Pb, Sr, and Nd) reveal an impressive compositional variation in the basalts recovered from Site 834. Major element compositions span almost the entire range observed in basalts from the modern axial systems of the Lau Basin, and variations are consistent with low-pressure fractionation of a mid-ocean-ridge-basalt (MORB)-like parent, in which plagioclase crystallization has been somewhat suppressed. Trace element compositions deviate from MORB in all but one unit (Unit 7) and show enrichments in large-ion-lithophile elements (LILEs) relative to high-field-strength elements (HFSEs) more typically associated with island-arc magmas. The Pb-isotope ratios define linear trends that extend from the field of Pacific MORB to highly radiogenic values similar to those observed in rocks from the northernmost islands of the Tofua Arc. The Sr-isotope compositions also show significant variation, and these too project from radiogenic values back into the field for Pacific MORB. The variations in key trace element and isotopic features are consistent with magma mixing between two relatively mafic melts: one represented by Pacific MORB, and the other by a magma similar to those erupted on 'Eua when it was part of the original Tongan arc, or perhaps members of the Lau Volcanic Group (LVG). Based on our model, the most radiogenic compositions (Units 2 and 8) represent approximately 50:50 mixtures of these MORB and arc end-members. Magma mixing requires that both components are simultaneously available, and implies that melts have not shown a compositional progression from arc-like to MORB-like with extension at this locality. Rather, it is apparent that essentially pristine MORB can erupt as one of the earliest products of backarc initiation. Indeed, repetition of isotopic and trace element signatures with depth suggests that eruptions have been triggered by periodic injections of fresh MORB melts into the source regions of these magmas. The slow and almost amagmatic extension of the original arc complex envisaged to explain the observed chemistry is also consistent with the horst-and-graben topography of the western side of the Lau Basin. Given the similarities between basalts erupted at the modern Lau Basin spreading centers and MORB from the Indian Ocean, the overwhelming evidence for involvement of mantle similar to Pacific MORB in the petrogenesis of basalts from Site 834 is a new and important observation. It indicates that the original arc was underlain by asthenospheric material derived from the Pacific mantle convection cell, and that this has somehow been replaced by Indian Ocean MORB during the last ~5.5 Ma.

Description: We use Nomarski differential interference contrast imaging to reveal the wealth of complex detail in plagioclase zoning for selected samples from Sites 834, 839, and 841. All sites contain some plagioclase with the very complex internal core zoning, convolute zoning, or very fine-scale euhedral oscillatory zoning of the sort generally considered typical of island-arc volcanic rocks. Plagioclase with contrasted zoning styles may coexist within a single lithologic unit or even within a single thin section. Especially notable is the presence of scattered plagioclase phenocrysts with complex zoning throughout Unit 7 in Hole 834B, which in other respects is relatively uniform in composition and appears to have had little or no differential sorting of crystals and liquid. Although our study is by no means comprehensive, it is sufficient to indicate that magmatic conditions have been variable during crystallization of these rocks, and mixing or at least minor contamination may be required to explain some of the relations observed. By analogy with experimental studies, it is possible that variations in water content, either over time or within different parts of a chamber or conduit system, have contributed to the observed contrasts in zoning.

Description: Experimental phase relations were used to assess the role of volatiles and crustal level fractional crystallization in the petrogenesis of lavas from Hole 839B in the central Lau Basin. Melting experiments were performed on Sample 135-839B-15R-2, 63-67 cm, at 1 atm, anhydrous, and 2 kbar, H2O-saturated (~6 wt% H2O in the melt) to determine the influence of variable pressure and H2O content on phase appearances, mineral chemistry, and liquid line of descent followed during crystallization. The effects of H2O are to depress the liquidus by ~100°C, and to suppress crystallization of plagioclase and orthopyroxene relative to olivine and high-Ca clinopyroxene. At 1 atm, anhydrous, olivine and plagioclase coexist near the liquidus, whereas orthopyroxene and then clinopyroxene appear with decreasing temperature. Crystallization of 50 wt% produces a residual liquid that is rich in FeO* (10.8 wt%) and poor in Al2O3 (13.6 wt%). At 2 kbar, H2O-saturated, the liquidus phases are olivine and chromian spinel, with high-Ca clinopyroxene appearing after ~10% crystallization. Plagioclase saturation is suppressed until ~20% crystallization has occurred. The residual liquid from 35 wt% crystallization is rich in AI2O3 (17.4 wt%), and poor in MgO (4.82 wt%); it contains moderate FeO* (8.2 wt%), and resembles the low-MgO andesites recovered from Hole 839B. On the basis of these experiments we conclude that the primitive lavas recovered from Hole 839B have experienced crystallization along the Ol + Cpx saturation boundary, under hydrous conditions (an ankaramitic liquid line of descent), and variable amounts of olivine and chromian spinel accumulation. The low-MgO andesites from Hole 839B are the products of hydrous fractional crystallization, at crustal pressures, of a parent magma similar to basaltic andesite Sample 135-839B-15R-2, 63-67 cm.

Description: The backarc glasses recovered during Ocean Drilling Program Leg 135 are unique among submarine tholeiitic glasses with respect to their oxygen fugacity and sulfur concentrations. Unlike mid-ocean-ridge basalt glasses, fO2 in these samples (inferred from ratios Fe3+/Fe2+) is high and variable, and S variations (90-1140 ppm) are not coupled with FeO concentration. Strong correlations occur between the alkali and alkaline-earth elements and both fO2 (positive correlations) and S concentrations (negative correlations). Correlations between fO2 and various trace elements are strongest for those elements with a known affinity for hydrous fluids (perhaps produced during slab dehydration), suggesting the presence of a hydrous fluid with high fO2 and high alkali and alkaline earth element concentrations in the Lau Basin mantle. Concentrations of S and fO2 are strongly correlated; high fO2 samples are characterized by low S in addition to high alkali and alkaline earth element concentrations. The negative correlations between S and these trace elements are not consistent with incompatible behavior of S during crystallization. Mass balance considerations indicate that the S concentrations cannot result simply from mixing between low-S and high-S sources. Furthermore, there is no relationship between S and other trace elements or isotope ratios that might indicate that the S variations reflect mixing processes. The S variations more likely reflect the fact that when silicate coexists with an S-rich vapor phase the solubility of S in the silicate melt is a function of fO2 and is at a minimum at the fO2 conditions recorded by these glasses. The absence of Fe-sulfides and the high and variable vesicle contents are consistent with the idea that S concentrations reflect silicate-vapor equilibria rather than silicate-sulfide equilibria (as in MORB). The low S contents of some samples, therefore, reflect the high fO2 of the supra-subduction zone environment rather than a low-S source component.

Description: The basement of Bougainville Guyot drilled at Site 831 consists of andesitic hyalobreccias derived from a submarine arc volcano. The volcanic sequence has been dated by K/Ar at approximately 37 Ma. The 121 m of andesitic hyalobreccias drilled in Hole 831B have been divided into five subunits of two types: one appears to be primary, and the other contains evidence of reworking and a subaerial clastic input. Variations are attributed to fluctuations in water depth. The distinctive hyalobreccias consist of andesitic blebs with chilled margins and peripheral fractures set in a chaotic greenish matrix that is mainly altered glass, with crystals similar to those in the blebs or clasts. Their formation is attributed to violent reaction of andesitic magma discharged into seawater, in perhaps the submarine equivalent of fire-fountaining. There was limited reworking by currents and debris flows on the flanks of the submarine volcano. The andesite shows no significant compositional variation in phenocryst phases throughout the drilled sequence and contains phenocrysts of plagioclase (An88-43), clinopyroxene (Ca44Mg46Fe10-Ca41Mg40Fe19), orthopyroxene (Ca4Mg79Fe17-Ca3Mg58Fe39), and titanomagnetite. There is a systematic change in volcanic composition with height in the section, from more mafic andesites at the base, to overlying more acid andesites, and strong evidence exists that magma mixing may have played a significant role in the genesis of these lavas. The andesites have affinities with the low-K arc tholeiite series. Trace element and isotopic systematics for these rocks indicate very minor involvement of a LILE- and 87Sr-enriched slab-derived fluid in their petrogenesis. This accords with the previous suggestion that Bougainville Guyot forms part of an Eocene proto-island arc developed along the southern side of the d'Entrecasteaux Zone, above a southward-dipping subduction zone.

Description: New petrographic and compositional data were reported for 143 samples of core recovered from Sites 832 and 833 during Ocean Drilling Program (ODP) Leg 134. Site 832 is located in the center and Site 833 is on the eastern edge of the North Aoba Basin, in the central part of the New Hebrides Island Arc. This basin is bounded on the east (Espiritu Santo and Malakula islands) and west (Pentecost and Maewo islands) by uplifted volcano-sedimentary ridges associated with collision of the d'Entrecasteaux Zone west of the arc. The currently active Central Belt volcanic front extends through the center of this basin and includes the shield volcanoes of Aoba, Ambrym, and Santa Maria islands. The oldest rocks recovered by drilling are the lithostratigraphic Unit VII Middle Miocene volcanic breccias in Hole 832B. Lava clasts are basaltic to andesitic, and the dominant phenocryst assemblage is plagioclase + augite + orthopyroxene + olivine. These clasts characteristically contain orthopyroxene, and show a low to medium K calc-alkaline differentiation trend. They are tentatively correlated with poorly documented Miocene calc-alkaline lavas and intrusives on adjacent Espiritu Santo Island, although this correlation demands that the measured K-Ar of 5.66 Ma for one clast is too young, due to alteration and Ar loss. Lava clasts in the Hole 832B Pliocene-Pleistocene sequence are mainly ankaramite or augite-rich basalt and basaltic andesite; two of the most evolved andesites have hornblende phenocrysts. These lavas vary from medium- to high-K compositions and are derived from a spectrum of parental magmas for which their LILE and HFSE contents show a broad inverse correlation with SiO2 contents. We hypothesize that this spectrum results from partial melting of an essentially similar mantle source, with the low-SiO2 high HFSE melts derived by lower degrees of partial melting probably at higher pressures than the high SiO2, low HFSE magmas. This same spectrum of compositions occurs on the adjacent Central Chain volcanoes of Aoba and Santa Maria, although the relatively high-HFSE series is known only from Aoba. Late Pliocene to Pleistocene lava breccias in Hole 833B contain volcanic clasts including ankaramite and augite + olivine + plagioclase-phyric basalt and rare hornblende andesite. These clasts are low-K compositions with flat REE patterns and have geochemical affinities quite different from those recovered from the central part of the basin (Hole 832B). Compositionally very similar lavas occur on Merelava volcano, 80 km north of Site 833, which sits on the edge of the juvenile Northern (Jean Charcot) Trough backarc basin that has been rifting the northern part of the New Hebrides Island Arc since 2-3 Ma. The basal sedimentary rocks in Hole 833B are intruded by a series of Middle Pliocene plagioclase + augite +/- olivine-phyric sills with characteristically high-K evolved basalt to andesite compositions, transitional to shoshonite. These are compositionally correlated with, though ~3 m.y. older than, the high-HFSE series described from Aoba. The calc-alkaline clasts in Unit VII of Hole 832B, correlated with similar lavas of Espiritu Santo Island further west, presumably were erupted before subduction polarity reversal perhaps 6-10 Ma. All other samples are younger than subduction reversal and were generated above the currently subduction slab. The preponderance in the North Aoba Basin and adjacent Central Chain islands of relatively high-K basaltic samples, some with transitional alkaline compositions, may reflect a response to collision of the d'Entrecasteaux Zone with the arc some 2-4 Ma. This may have modified the thermal structure of the subduction zone, driving magma generation processes to deeper levels than are present normally along the reminder of the New Hebrides Island Arc.

Description: This paper discusses the Paleobathymetric and paleoenvironmental history of the New Hebrides Island Arc and North d'Entrecasteaux Ridge during Cenozoic time based on benthic foraminiferal and sedimentological data. Oligocene and Pliocene to Pleistocene benthic foraminiferal assemblages from Sites 827, 828, 829, and 832 of Ocean Drilling Program (ODP) Leg 134 (Vanuatu) are examined by means of Q-mode factor analysis. The results of this analysis recognize the following bathymetrically significant benthic foraminiferal biofacies: (1) Globocassidulina subglobosa biofacies and Bulimina aculeata-Bolivinita quadrilatera biofacies representing the upper bathyal zone (600-1500 m); (2) Gavelinopsis praegeri-Cibicides wuellerstorfi biofacies, indicating the Pacific Intermediate Water (water depth between 1500 and 2400 m); (3) Tosaia hanzawai-Globocassidulina muloccensis biofacies, Valvulineria gunjii biofacies, and the Melonis barleeanus-Melonis sphaeroides biofacies, which characterize the lower bathyal zone; (4) the Nuttallides umbonifera biofacies, which characterizes the interval between the lysocline (approximately 3500 m) and the carbonate compensation depth (approximately 4500 m); and (5) the Rhabdammina abyssorum biofacies representing the abyssal zone below the carbonate compensation depth. Benthic foraminiferal patterns are used to construct Paleobathymetric and paleogeographic profiles of the New Hebrides Island Arc and North d'Entrecasteaux Ridge for the following age boundaries: late Miocene/Pliocene, early/late Pliocene, Pliocene/Pleistocene, and Pleistocene/Holocene.

Description: In the cores obtained during Leg 134 of the Ocean Drilling Program, radiolarians occur intermittently and usually in a poor state of preservation, apparently as a result of the region having been at or near the boundary between the equatorial current system and the south-central Pacific water mass during most of the Cenozoic. A few well-preserved assemblages provide a record of the Quaternary forms, and some displaced middle and lower Eocene clasts preserve a record of radiolarians near that subepochal boundary. There are less satisfactory records of middle Miocene and early Miocene to late Oligocene forms.

Description: Sedimentological and organic geochemical investigations of sediments of Site 798 give important information about the paleoenvironmental and paleoceanographic conditions of the Japan Sea. Distinct dark/light cycles from the uppermost 37 m of the record (i.e. the last 200,000 years) were sampled in detail and analyzed for TOC content, C/N-ratios and hydrogen index values to characterize the organic material. A subdivision into three types of cycles was possible, representing different environmental conditions during formation of the dark/light rhythms. Additionally, the data were correlated with the global oxygen isotope curve and biogenic opal data to define the stratigraphic framework and stage boundaries. The results indicate that glacial intervals are generally dominated by a very distinctive cyclicity whereas interglacial sediments are more homogenous and cycles less pronounced. Glacial sea-level lowstands probably lead to a stratification of the water column and anoxic sedimentary conditions (and, thus, distinct dark/light cyclicity with an enrichment of marine organic carbon in the dark part) while the well-mixed Japan Sea of the interglacial periods barely reached oxygen-deficient levels although high productivity caused the enhanced oxygen demand due to the decay of marine organic material. Dilution by siliciclastic material and supply of terrigenous organic matter may occasionally control the organic carbon fluctuations.

Description: The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122-760A, ODP 122-762B, BMR96GC21 and RC9-150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The delta13C trends of Cibicidoides wuellerstorfi, and differences between the delta13C trends of planktics (Globigerinoides sacculifer) and benthics (C. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in delta13C, and strong delta13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, delta13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated by Uvigerina peregrina, another dominated by U. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and of Epistominella exigua and Bulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum delta13C values of C. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.

Description: Fluctuations in the 87Sr/86Sr ratio of seawater accompanying glacial/interglacial climate changes have been reported in recent studies but remain highly controversial. To investigate these potential fluctuations we present very high precision (13 ppm) 87Sr/86Sr measurements on planktonic foraminifera from Indian Ocean and Pacific cores. 87Sr/86Sr ratios from three different foraminiferal species are indistinguishable from one another in both core-top and 50 ka samples, demonstrating that changes due to diagenesis or contamination do not influence the measurements. Average 87Sr/86Sr ratios for Pacific Ocean and Indian Ocean samples are also indistinguishable at three intervals (core-top, 50 ka and 300-370 ka), demonstrating that the oceans have remained well mixed with respect to Sr. Also, 87Sr/86Sr ratios are not affected by changes in the precleaning of samples. Measurements from Pacific core V28-238, that used in the study of Dia et al. (1992, doi:10.1038/356786a0), do not reproduce the cycles seen in the previous study and are statistically well explained by a linear increase in the seawater 87Sr/86Sr ratio. It seems likely that an analytical artefact caused the cycles in the previous study. Measurements from two key sections of core from ODP site 758, that used in the study of Clemens et al. (1993, doi:10.1038/363607a0), do not reproduce the largest shifts seen in the previous study, despite the considerably better precision reported here. The apparent cyclicity in the Clemens et al. (1993) data is also suggested to be due to an unknown analytical artefact. The 48 measurements presented here are a statistically good fit to a straight line defined by Delta 87Sr (ppm) = -0.0613 * age (ka). At the 13 ppm level of precision no evidence for a glacial/interglacial variation in the seawater 87Sr/86Sr ratio is seen. Glacial/interglacial variation may still exist but this data constraints its maximum amplitude to 6-9 ppm, which would correspond to changes in the riverine Sr flux of c. 30%.

Description: A reconstruction of late Pleistocene surface water carbon isotopic (delta13C) variability is presented from Ocean Drilling Program (ODP) site 769 in the Sulu Sea in the western tropical Pacific. The Sulu Sea is a shallowly silled back arc basin with a maximum sill depth of 420 m. Site 769 was drilled on a bathymetric high in 3643 m of water and has average late Pleistocene sedimentation rates of 8.5 cm/kyr. The oxygen isotope record (delta18O) of Globigerinoides ruber at site 769 shows a strong correlation with the SPECMAP stacked delta18O record, attesting to the continuity of sediment archive at the site. Surface delta13C displays consistent glacial-interglacial variability which averages ~0.9 per mil and has varied from 0.75 to 1.1 per mil over the last 800 kyr. Comparison to surface water delta13C records in the South China Sea and western tropical Pacific suggests that the glacial-interglacial surface delta13C variability is regional in scale. Planktonic delta13C data from ODP site 677 in the eastern Pacific is also coherent with the site 769. Additionally, we have found that the site 769 surface delta13C record is coherent at periods of 100 and 41 kyr with deepwater delta13C records from the Pacific. The highest correlation occurs with the deep eastern Pacific, where benthic delta13C data from cores RC13-110 and ODP site 677 closely match the Sulu Sea surface water record. We evaluate several possible controls of surface water delta13C in the Sulu Sea that may explain the coherent timing with Pacific deepwater delta13C records. These include variations in terrestrial organic matter flux to the basin, the upwelling of subsurface water and productivity changes, and the influx of western Pacific intermediate water to the Sulu Sea. Our preferred explanation involves a region of upper intermediate water upwelling in the far western Pacific which has been shown to outgas CO2 from subsurface waters into surface waters. Upwelling also occurs in the area of Panama Basin site 677. These equatorial upwelling zones could potentially provide a route by which Pacific intermediate water can modulate the delta13C composition of certain Pacific surface water locations. Future reconstructions of late Pleistocene surface water delta13C variability in the western Pacific and Indonesian seas will be required to further evaluate the source of the glacial-interglacial surface water delta13C change.

Description: We undertook a quantitative study of Thecosomata shells (pelagic gastropods) and their remains in Quaternary foraminiferal oozes deposited on the tilted calcareous platform of the Bougainville Guyot (Hole 831 A), and in the late Quaternary volcanic siltstones, claystones and sandy interbeds on the upper forearc slope of the central New Hebrides Island Arc (Hole 830A). The distribution of the species is based on the identification of adult shells, juvenile stages, protoconchs, and characteristic shell fragments. By studying thecosomatous shells using a scanning electron microscope (SEM), we were able to specify the fine microstructure of the coiled Limacina inflata and compare it with the rod-type crossed-lamellar structure of some other Limacina species, as well as with the helical structure of the Cavoliniidae.

Description: In-situ proton-microprobe analyses are presented for glasses, plagioclases, pyroxenes, olivines, and spinels in eleven samples from Sites 834-836, 839, and 841 (vitrophyric rhyolite), plus a Tongan dacite. Elements analyzed are Mn, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Pb, and Sn (in spinels only). The data are used to calculate two sets of partition coefficients, one set based on the ratio of element in mineral/element in coexisting glass. The second set of coefficients, thought to be more robust, is corrected by application of the Rayleigh fractionation equations, which requires additional use of modal data. Data are presented for phenocryst core-rim phases and microphenocryst-groundmass phases from a few samples. Comparison with published coefficients reveals an overall consistency with those presented here, but with some notable anomalies. Examples are relatively high Zr values for pyroxenes and abnormally low Mn values in olivines and clinopyroxenes from Site 839 lavas. Some anomalies may reflect kinetic effects, but interpretation of the coefficients is complicated, especially in olivines from Sites 836 and 839, by possible crystal-liquid disequilibrium resulting from mixing processes.

Description: The Cenozoic sediments sampled in ODP Leg 104 on the Vøring Plateau show a distinct variability of the total organic carbon content (TOC) and the accumulation rates of TOC. Based on the geochemical and organic-petrographic characterization of the sedimentary organic matter (OM), the allochthonous and autochthonous proportion of the OM could be quantified. The results clearly demonstrate that high TOC percentages and TOC accumulation rates in Cenozoic sediment sections display a generally high input of allochthonous organic matter. Oxidized and partly well-rounded organic particles built up the main portion of OM within the Miocene, TOC-rich sediments. The most probable source of this oxidized OM are reworked sediments from the Scandinavian shelf. Changes in the input of these organic particles are to some degree correlative with sea-level changes. The Cenozoic accumulation of autochthonous OM is low and does not reveal a clear variation during the Miocene and early Pliocene. In spite of a high accumulation rate of biogenic opal during the Early Miocene, the accumulation rate of autochthonous TOC is low. The autochthonous particle assemblage is dominated by relatively inert OM, like dinoflagellate cysts. This points to an intensive biological and/or early diagenetic degradation of the marine OM under well oxidized bottom water conditions during the last 23 Myr. Nevertheless, a continuation of marine OM degradation during later stages of diagenesis cannot be excluded. A prominent dominance of allochthonous OM over autochthonous is documented with the beginning of the Pliocene. At 2.45 Ma the episodic occurrence of ice-rafted, thermally mature OM reflects the onset of the glacial erosion of Mesozoic, coal and black shale bearing sediments on the Scandinavian and Barents Sea shelves. The first occurrence of these, in view of the actual burial depth, thermally overmature OM particles is, therefore, a marker for the beginning of the strong Scandinavian glaciation and the advance of the glacial front toward the shelves.

Description: Structural and paleomagnetic studies of Ocean Drilling Program (ODP) cores are severely hampered by the fact that the recovered cores can rarely be reoriented with any degree of confidence. Although several techniques for orienting cores have been used, none is without limitations. Core orientation devices used during drilling are expensive to operate and are usually unreliable. A common alternative, used when no core orientation tools are available, is to assume that the horizontal component of the magnetization direction of the core pointed north, and thus that features may be oriented relative to this direction. However, this neglects the effects of secular variations in the Earth's magnetic field and of later tectonic rotation on the sample. In this paper we outline a technique for core reorientation that matches distinctive inclined planar features measured on the core with their images on Formation MicroScanner and Borehole Televiewer wireline logs. It allows reorientation of structural and magnetic data, and hence the possibility of detecting vertical axis tectonic rotations using Paleomagnetism. Its methodology is described and an example, from ODP Leg 135, presented to illustrate its application.

Description: In low and middle latitudes, the Cretaceous/Tertiary boundary is marked by a sudden and pronounced decrease in d13C values of near-surface-water carbonates and a reduction in the surface-to-bottom d13C gradient. These isotopic data have been interpreted as evidence of a decline in surface-water productivity that was responsible for the extinction of many planktic foraminiferal species and other marine organisms at or near the K/T boundary. We present planktic and benthic foraminiferal isotopic data from two almost biostratigraphically complete sections at Ocean Drilling Program Site 738 in the antarctic Indian Ocean and at Nye Kløv in Denmark. These data suggest that planktic carbonate d13C values in high latitudes may not have decreased dramatically at the K/T boundary; thus, surface-water productivity may not have been reduced as much as in low and middle latitudes. Comparison of the records of Site 738 with those of ODP Sites 690 and 750 indicates a pronounced decline in d13C values of planktic and benthic foraminifera and fine-fraction/bulk carbonate ~200 000 yr after the K/T boundary. This reflects a regional shift in the carbon isotopic composition of oceanic total dissolved carbon (TDC) and correlates with a similar change in benthic foraminiferal d13C values at mid- and low-latitude Deep Sea Drilling Project Sites 527 and 577. This oceanographic event was followed by the ecosystem's global recovery ~500 000 yr after the K/T boundary. These data suggest that the environmental effects of the K/T boundary may have been less severe in the high-latitude oceans than in tropical and subtropical regions.

Description: The K-Ar ages from the basaltic rocks of Leg 134 range from Miocene to Holocene (Table 1). Samples were selected in consultation with shipboard scientists; choice of the material from the forearc sites was very limited and confined to clasts. There was a wider choice of material from the sill at Site 833 in the North Aoba Basin.